The invention concerns a method for determining the threshold of deposition of heavy fractions, i.e., according to the invention, generally asphaltenic polar fractions, that are contained in the dissolved and/or stable colloidal state in a liquid hydrocarbon-containing fluid. It further concerns a device for implementing said method.
Many petroleum crudes, notably those referred to as asphaltenic petroleum crudes, are liquid hydrocarbon-containing fluids that contain more or less large quantities of heavy fractions in the dissolved and/or stable colloidal state under the pressure and temperature conditions which said fluids are subjected to. When these pressure and/or temperature conditions vary, notably when the pressure decreases, the heavy fractions contained in these fluids tend to flocculate and to settle in the formation surrounding the wells, in the wells and in the production and transfer facilities intended for said fluids. Thus, when a hydrocarbon reservoir containing heavy fractions is developed, the stability of these fractions decreases generally before the bubble point is reached. When the saturation threshold is reached, the heavy fractions flocculate and settle, which can lead to clogging of the porous media and formation of plugs likely to cause severe damage to the production wells and to the surface installations.
For petroleum producers, who have to extract and to convey, through production wells and pipe networks, liquid hydrocarbon-containing fluids consisting of petroleum crudes containing heavy fractions, for example asphaltenic petroleum crudes, coming from production fields, it is therefore important to precisely know the pressure thresholds above which heavy fractions settle, so as to carry out production and transfer of said fluids under pressure and temperature conditions that prevent heavy fraction deposition in the facilities, or to provide a suitable treatment.
There are various known methods for determining the deposition threshold of heavy fractions, notably asphaltenes, contained in liquid hydrocarbon-containing fluids consisting of petroleum crudes. These methods are most often optical light transmission or scattering methods, conductimetric methods, or viscometric methods.
The aforementioned methods involve detection of the variation of a physical quantity, for example the absorption coefficient or absorbance of light beams in the visible range or in the infrared, the electrical conductivity, or the viscosity, which is due to a change in the consistency of the fluid as a result of the flocculation and the deposition of the heavy fractions.
A major drawback of such methods is that they are not very selective insofar as the variation of the physical quantity measured cannot be always readily connected to the flocculation and the deposition of heavy fractions, and they are not always sensitive to the deposition of a small amount of such fractions. Some methods, such as the measurement of the absorbance in the infrared, are very sensitive, but difficult to implement under reservoir conditions.
Furthermore, since these methods are often used in the laboratory, the question which must also be considered concerns the representativeness of the samples on which the physical quantity measurements are performed. In fact, for a sample to be representative of the sampled fluid, it is necessary to maintain this sample under the pressure and temperature conditions that prevail for the sampled fluid, for example a reservoir fluid, throughout the sampling, sample transport and storage operations that are carried out prior to measurement.
The invention provides a method for determining the threshold of deposition of heavy fractions, notably asphaltenes, contained in the dissolved and/or stable colloidal state in a liquid hydrocarbon-containing fluid, based on the creation of an increasingly great pressure drop linked with the flow, at increasing flow rate, of a sample of said fluid through a capillary passage. Such a method has an improved selectivity and sensitivity in relation to the aforementioned prior methods and it allows to overcome the drawbacks of these methods. Furthermore, the method according to the invention can be implemented in situ in wells producing fluids, notably asphaltenic fluids, which also provides an answer to the problem of the representativeness of the samples measured.
The method according to the invention for determining the threshold of deposition of heavy fractions contained in the dissolved and/or stable colloidal state in a liquid, hydrocarbon-containing fluid is characterized in that it consists in:
bringing a sample of said fluid, maintained at a constant pressure and temperature such that the heavy fractions present in said fluid sample are in the dissolved and/or stable colloidal state, to the inlet of a capillary passage comprising an inlet and an outlet, and likely to generate a pressure drop between the inlet and the outlet that is at least equal to the difference between the pressure of the fluid sample and the bubble-point pressure of said sample,
establishing an initial fluid pressure at the outlet of the capillary passage that is substantially equal to the pressure of the fluid sample brought to the inlet of said passage,
generating, by operating at a constant temperature substantially equal to the temperature of the fluid sample, a flow of the liquid fluid, at an increasing flow rate, through the capillary passage, until the pressure at the outlet of the capillary passage has dropped from the original pressure to a predetermined pressure above the bubble-point pressure of the sample,
measuring a difference xcex94P between the pressure of the fluid at the inlet of the capillary passage and pressure Ps of the fluid at the outlet of said capillary passage and/or said outlet pressure Ps, as well as a quantity D representative of the flow of liquid flowing through the capillary passage,
detecting a significant shift in the variation of one of quantities xcex94P and D as a function of time or as a function of the other quantity or of a quantity representative of this other quantity, and
defining as the deposition pressure of the heavy fractions at the operating temperature, for the liquid hydrocarbon-containing fluid subjected to the test, the fluid pressure at the outlet of the capillary passage which corresponds to a start of said shift, this pressure characterizing the threshold of deposition of the heavy fractions for said fluid.
In particular, the last two aforementioned stages of the method according to the invention can be carried out by representing, in form of a curve, the variation of one of the quantities xcex94P and D as a function of time or as a function of the other quantity or of a quantity representative of this other quantity, and by defining as the deposition pressure of the heavy fractions at the operating temperature, for the liquid hydrocarbon-containing fluid subjected to the test, the fluid pressure at the outlet of the capillary passage which corresponds to the start of a change in the slope of the variation curve of quantity xcex94P or of quantity D.
According to an embodiment, the capillary passage is filled with a stationary phase that can be selected, for example, from among the stationary phases used in high-pressure liquid chromatography.
Whether filled with a stationary phase or free from such a phase, the capillary passage has a void space advantageously ranging between 1 xcexcl and 5000 xcexcl, and more particularly between 10 xcexcl and 100 xcexcl.
The pressure of the liquid sample brought to the inlet of the capillary passage can vary quite widely and it can notably range between 5 bars and 1500 bars.
The volume of liquid sample sweeping the capillary passage during operation advantageously represents 50 to 500,000, preferably 10,000 to 100,000 times the void space of the capillary passage.
In a first embodiment of the method according to the invention, the liquid flow is generated at an increasing flow rate through the capillary passage by decreasing pressure Ps at the outlet of the capillary passage, continuously or in stages, according to a predetermined profile as a function of time, so as to change from the initial outlet pressure to the predetermined pressure above the bubble-point pressure of the sample, and the variation of quantity D is recorded, which is representative of the flow of liquid flowing through the capillary passage, as a function of time, when pressure Ps decreases in stages, or as a function of xcex94P or of Ps, when pressure Ps decreases continuously, so as to produce the curve from which the deposition pressure of the heavy fractions is defined.
The pressure decrease at the outlet of the capillary passage can be generated in particular at a rate ranging between 0.1 and 50 bar/minute, and more particularly between 0.5 and 10 bar/minute.
In a second embodiment of the method according to the invention, the liquid flow is generated at an increasing flow rate through the capillary passage by discharging the liquid at the outlet of said capillary passage, with an increasing flow rate, continuously. or in stages, according to a predetermined law as a function of time, so as to change from the initial outlet pressure to the predetermined pressure above the bubble-point pressure of the sample, and the variation of quantity xcex94P is recorded, which is representative of the difference between the fluid pressure at the inlet of the capillary passage and pressure Ps of the fluid at the outlet of said capillary passage, as a function of time, when the flow of liquid discharged increases in stages, or as a function of quantity D when said flow rate increases continuously, so as to produce the curve from which the deposition pressure of the heavy fractions is defined.
On the curves used to define the deposition pressure of the heavy fractions, it is also possible to measure the rate of variation of the slope of the portion of said curves starting at the point that corresponds to the deposition pressure of the heavy fractions, and said rate gives an indication of the rate of deposition of said heavy fractions.
The method according to the invention can also be used to evaluate the efficiency of an additive intended to inhibit or to retard the deposition of heavy fractions contained in a liquid hydrocarbon-containing fluid extracted from a reservoir. Determined amounts of additive are therefore injected into the hydrocarbon-containing fluid to be studied prior to introducing said fluid into the capillary passage, by operating under pressure and temperature conditions representative of the reservoir conditions, and the method is then implemented as mentioned above so as to produce the curves used for defining the deposition pressure of the heavy fractions. The efficiency of the additive can then be evaluated at two levels, (i) at the level of the displacement of the point of the curves corresponding to the deposition pressure of the heavy fractions and (ii) at the level of the rate of deposition measured by means of the rate of variation of the slope of the curve portion starting at said point corresponding to the deposition pressure of said heavy fractions.
According to a particular embodiment, the method according to the invention is implemented within the liquid hydrocarbon-containing fluid comprising heavy fractions, notably asphaltenes, in the dissolved and/or stable colloidal state, the inlet of the capillary passage being then directly in contact with said fluid.
A device for implementing the method according to the invention is characterized in that it comprises:
an inlet chamber for a liquid, equipped with delivery means and means for maintaining said liquid at a constant pressure in said chamber,
an outlet chamber equipped with means providing generation of a flow of liquid at an increasing flow rate, and allowing to apply to said flow a rate or a pressure that varies continuously or in stages, and provided with means for measuring the pressure of the liquid in this chamber and a quantity D representative of the rate of the flowing liquid,
a capillary line provided with an inlet and an outlet, and connected by its inlet to the inlet chamber and, by its outlet, to the outlet chamber, said capillary line being able to generate a pressure drop between its inlet and its outlet that is at least equal to the difference between the pressure of the liquid in the inlet chamber and the bubble-point pressure of this liquid.
The device according to the invention can also comprise means for maintaining the elements at constant temperature.
The capillary line can be filled with a stationary phase that can be selected as mentioned above for the capillary passage.
Whether filled with a stationary phase or free from such a phase, the capillary line has a void space that advantageously ranges between 1 xcexcl and 5000 xcexcl, more particularly between 10 xcexcl and 100 xcexcl.
The means for maintaining the liquid at a constant pressure in the inlet chamber can consist, for example, of a piston pump operated at said constant pressure.
According to a particular embodiment, which is especially interesting for implementing the method within the liquid hydrocarbon-containing fluid, the inlet chamber of the device has an open end opposite the inlet of the capillary line, which serves as delivery means and means for maintaining the liquid contained in said chamber at a constant pressure, when this chamber is immersed in the liquid hydrocarbon-containing fluid. With this embodiment, a filter is advantageously interposed between the open end of the inlet chamber and the inlet of the capillary line to prevent solid particles from being carried along.
The means that the outlet chamber is equipped with and which provide generation of a flow of liquid at increasing flow rate, continuously or in stages, can consist of (i) liquid draw-off means with a controlled downstream flow rate that increases continuously or in stages, for example a controlled and continuously or stepwise increasing downstream flow rate pump, or of (ii) draw-off means with a controlled and continuously or stepwise decreasing downstream pressure, for example, a throttling valve or a controlled and continuously or stepwise decreasing pressure pump.
In particular, the outlet chamber equipped with draw-off means for a liquid having a continuously or stepwise increasing flow rate consists of a cylindrical chamber in which a piston slides and moves in translation either under the action of a driving system intended to work at a speed that increases continuously or in stages, or by application of a pressure that decreases continuously or in stages. Said chamber is equipped with a pressure detector and with means for measuring the rate of displacement of the piston, said rate being a function of the flow of liquid discharged and, for example, proportional to said flow rate.
According to another embodiment, the outlet chamber and its equipments can also consist of the downstream portion of the capillary line, equipped with a pressure regulating valve, controlled by a variable control regulator, with a pressure detector connected to the regulator and with a flowmeter, arranged upstream from the valve, the inlet of said valve representing the outlet of the capillary line.
The device can also comprise additional means associated with the pressure measuring means in the outlet chamber and with the means for measuring quantity D representative of the flow of liquid and arranged to detect a significant shift in the variation of one of quantities xcex94P and D as a function of time or as a function of the other quantity or of a quantity representative of this other quantity, quantity xcex94P representing the pressure difference between the pressures in the inlet and outlet chambers, i.e. the pressure difference between the inlet and the outlet of the capillary line.
In particular, said additional means associated with the pressure measuring means in the outlet chamber and with the means for measuring quantity D representative of the flow of liquid can be recording means producing records, as a function of time, of said pressure and quantity D representative of the flow rate and/or producing the variation curve of quantity xcex94P as a function of said quantity D or as a function of time, or the variation curve of said quantity D as a function of time or as a function of xcex94P or of pressure Ps in the outlet chamber.
The device according to the invention, for which the inlet chamber is provided with an open end opposite the inlet of the capillary line and possibly comprises a filter interposed between said open end and the inlet of the capillary line, can be advantageously included in a bottomhole sample taker that can be lowered into a well producing the hydrocarbon-containing fluid including heavy fractions, notably asphaltenes, in such a way that the open end of the inlet chamber is visible.